How do I deal with someone who's been shitting in front of my house?

I've been seeing poop in a path which is connected to the gates of my house. This has happened 3 times now over 1 or 2 weeks. When I saw the poop first, I thought it must be a dog. But everytime it dries (every 3 or 4 days), a mysterious pile of new shit is dropped on the old shit. It's (very) black, very smelly and in the EXACT same spot!! Right on top of the dried shit!! I'm convinced it's a human being. I've decided I'm gonna hide in some bushes nearby and catch them doing the deed. But what do I do then? What if they don't care and continue doing it? How do I make them regret it without getting into trouble? It'd be nice if they couldn't tell it's me.